The present invention discloses a line voltage feed-forward circuit for a power factor correction controller or other integrated circuit and, more particularly, pertains to a quantized, voltage feed-forward device that eliminates low-frequency filtering and provides a fast response to line voltage changes and to methods and systems using the same.
Power factor correction (PFC) refers to a process to offset or improve the undesirable effects of non-linear electric loads that contribute to a power factor (PF) that is less than unity. In pertinent part, these effects involve the phase angle between the voltage and the harmonic content of the current. When the voltage and current are in phase, the PF is unity, but when the voltage and current are not in phase the PF is some value less than 1.
PFC controllers often rely on feed-forward of some scaled function of the alternating current (AC) line voltage to stabilize the input-to-output gain of the voltage loop. Conventionally, the scaled function of the AC line voltage corresponds to the root-mean-square (rms) level of the input voltage (Vrms). For example, typically, the input Vrms capability of much of the world's electronic equipment ranges between about 264 volts and about 85 volts, which is roughly a 3-to-1 range. The variation of control loop gain under these conditions, however, is about 10-to-1. By incorporating Vrms feed-forward into the control loop function, loop gain is stabilized, making frequency compensation easier and loop response to disturbances faster.
Conventional voltage feed-forward (VFF) circuits used in connection with analog signals typically include diodes and an RC network, respectively, to rectify and filter the sinusoidal line voltage. More particularly, conventional VFF circuits represent the input Vrms level of the line voltage by deriving the near-DC voltage level from a scaled waveform proportional to the rectified input voltage after the voltage has been averaged using a low-pass filter (LPF). Controller circuitry then mathematically squares the value of the voltage and further scales the squared term to determine the magnitude of the input current reference waveform controlled by the PFC integrated circuit.
Problematically, if the RC network is adapted to provide the least amount of filtering, remnant, low-frequency (e.g., twice the line frequency) AC signals are still present on the near-DC voltage in the waveform. Even though the magnitude of the low-frequency AC signals may only be measured in milli-volts (“ripple”), harmonic distortion, including 3rd-order harmonic distortion, is introduced into the controlled AC reference waveform.
Alternatively, to substantially eliminate 3rd-order harmonic distortion, the RC network can be adapted to provide “heavier” filtering. Disadvantageously, “heavier” filters are slower and operate at lower frequencies, which may cause the AC reference signal to lag changes in the AC input by several cycles before the input current reference waveform reaches a steady-state. Signal lag, hence, can result in output over- and under-voltage conditions, which cause other detrimental consequences.
Making a trade-off between acceptable total harmonic distortion (THD) and a fast response to AC line transients is, therefore, necessary. Accordingly, it would be desirable to provide a quantized, voltage feed-forward (QVFF) device that eliminates the need to remove low-frequency harmonic content using RC filtering networks. Furthermore, it would be desirable to provide a QVFF device that can adjust the input current reference waveform within every half-cycle. It also would be desirable to provide a QVFF device that provides a fast response to line voltage changes and that removes ripple-induced, 3rd-order harmonic distortion.